It is known that many thermoplastic polymers can be formed into shaped objects by various sheet metal forming techniques such as deep drawing, stretch forming, stamping, forging, cold extrusion, etc. These forming processes are economically very attractive because it is possible to achieve rapid production rates, e.g., rates that exceed one item a minute or even faster.
Unfortunately, reinforced thermoplastic parts prepared by these processes tend to exhibit poor surface characteristics such that they are unacceptable for large automotive exterior surfaces. In order to overcome these deficiencies, sheets and processes such as those disclosed in U.S. Pat. No. 3,684,645 and copending application Ser. No. 564,019, filed Apr. 1, 1975 now U.S. Pat. No. 4,044,188, entitled Stampable Thermoplastic Sheet Reinforced with Multilength Fiber, were developed.
Generally in this art, a thermoplastic sheet, reinforced with a glass mat type of reinforcement is preheated in an oven to above the softening point of the resin. The heated blank is transferred to the matched metal dies of a stamping press, and stamped in a modified mechanical or rapid-closing hydraulic press or the like. The formed part is then removed or ejected from the matched dies. The residence time in the mold is 20-80 seconds or less. The forming process can be separated into the following distinct stages.
1. Preheat of sheet or blank. PA1 2. Transfer to the rapid-stamping press. PA1 3. Stamping under pressure for a predetermined time. PA1 4. Removal or ejection from the press.
In stage 1, the composite sheet or blank is heated in a radiant, dielectric, infrared, convection, or vacuum oven or combination of ovens or similar heating source to a temperature above the melting point but below the decomposition point of the thermoplastic resin component of the blank.
In stage 2, the hot blank is transferred to a mold placed in a stamping press, wherein the mold or set of dies can impart the desired configuration of the final product to the sheet or blank. The mold is maintained at a temperature between room temperature (23.degree. C.) and approximately 160.degree. C. or more, depending on the polymer constituent of the sheet and upon the desired stamping characteristics.
In stage 3, the press is rapidly closed for a period of time sufficient to cause the blank to conform to the exact shape of the mold and to cool and/or crystallize sufficiently to allow part removal without distortion.
In stage 4, the molded article is removed from the mold.
It will be recognized that stamping parameters such as pressure requirements, residence time in the mold, preheat temperature, mold temperature, etc., are dependent upon the sheet composition, thickness, part complexity, etc.
Glass mat reinforced sheets of this kind as provided heretofore also suffer from certain disadvantages. Specifically, the following deficiencies have been encountered in prior art compositions:
Surface Finish: Prior art compositions usually contain 30 to 50% by weight of glass mat. Such an amount is required in order to impart high mechanical strength properties, high modulus, and sheet integrity during the preheat and transfer stages of the rapid stamping cycle.
A substantial amount of glass mat is needed not only to yield good mechanical strength properties in the final stamped part, but also to retain sheet integrity during the preheat and transfer stages of the rapid stamping process. However, high concentration of glass mat of quantity needed to provide the necessary mechanical strength and to retain the integrity of the sheet when it is heated tends to produce a poor surface finish on the final stamped part. For application such as automotive exterior parts (fenders, hoods), appliance housings, furniture components, etc., a smooth, imperfection-free surface is mandatory.
Prior attempts to remedy this problem as set forth in above noted U.S. Pat. No. 3,684,645 and copending application Ser. No. 564,019, have led to sheet, which though apparently smooth, fails to pass the visual inspection tests to which finished painted parts are normally subjected in the automotive industry. One such test is to view the reflection of a fluorescent light in the painted part to determine if the surface is wavy, albeit smooth. If the surface is wavy, the part is rejected. Also, a poblem has been found with "show-through" of the long glass mat which is again a visual test applied to the painted part.